1. Field of the Invention
The present invention relates to a measuring circuit using a photomultiplier, and more particularly to a photomultiplier control circuit for stabilizing the gain of the photomultiplier.
2. Description of Related Art
A photomultiplier, in which a cathode receives an incident light to convert it to electrons which are amplified by a multiplicity of dynodes and then taken out from an anode (plate) in the form of a current or electric charges, is extensively utilized in measuring a very small amount of light because it has a high degree of amplification.
When the output of a photomultiplier is an analog current, the gain of the photomultiplier is varied with abrupt changes in the level of the incident light upon the photomultiplier, as shown in FIG. 1. In other words, the gain is lowered when the incident light is at a high level, while the gain is increased when the incident light is at a low level. This is supposed to be mainly attributable to a fatigue and restoration phenomenon of dynodes, which is also referred to as a phenomenon of hysterisis because the current gain is determined by the previous operation hysterisis.
When attempting to precisely measure the incident light, such fluctuations in the gain can not be ignored. There is a method capable of compensating the gain of the photomultiplier when the incident light can be chopped into the form of light pulses, but it is impossible to compensate the gain when the incident light is continuously input in the form of a direct current. Therefore, in a thickness gauge of the type wherein radiation of a certain dosage is emitted from a radiation source and wherein the part of the radiation having passed an inspected substance is converted to a light by a scintilator, for example, which light is detected by a photomultiplier to measure a thickness of the inspected substance, the foregoing gain fluctuations give rise to a substantial problem in an attempt to obtain a high accuracy of measurement. This problem occurs when the inspected object is a plate material flowing over the manufacturing line and when plates having a thickness different from that of the previous ones begin to flow over the line.